1. Field of the Invention
The present invention relates to the breeding of rice seed and, in particular, to the identification of an inbred rice seed designated A0044 and its counterpart fertile maintainer line designated B0044. The present invention also relates to the plants produced by the cultivation of inbred rice seed designated A0044, as well as plants having A0044 as a parent. Additionally, the invention is directed to methods of maintaining the male-sterile line A0044 and of producing hybrids with inbred line A0044 as the female parent.
2. Description of Related Art
The goal of plant breeding is to combine in a single variety or hybrid various traits of agronomic or commercial interest. For field crops, these traits may include resistance to diseases and insects, tolerance to heat and drought, reduced time to crop maturity, greater yield, and improved agronomic characteristics. As a consequence of the mechanical harvesting of many crops, uniformity of plant and seed traits such as germination, stand establishment, growth rate, maturity, and fruit size are also goals of most modern plant breeding programs.
Field crops are bred through techniques that take advantage of the method of pollination of particular plant species. A plant is self-pollinated if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant.
Plants that have been self-pollinated and selected for type for many generations become homozygous at virtually all gene loci and produce uniform populations of true breeding progeny. A cross between two homozygous but genetically different lines would then produce a uniform population of hybrid plants that may be heterozygous for many gene loci. In contrast a cross of two plants each of which is heterozygous at a number of gene loci will produce a mixed population of hybrid plants that differ genetically and will not be uniform.
Rice plants (Otyza saliva) have perfect flowers, that is they have both male and female structures within the same flower, and rice plants are naturally self-pollinated. Breeding efforts for improvement of the rice crop have reflected this biological fact. Rice plants, left to themselves, will self-pollinate and will therefore be substantially homozygous at most gene loci.
The development of improved pure or true-breeding rice lines or varieties involves crossing two lines having different desirable characteristics by manually transferring pollen from one plant to another, and during subsequent generations, selecting plants which appear to have some combination of desirable characteristics from each parent. Pedigree breeding, which is the development of homogeneous lines during the inbreeding of populations of both self- and cross-pollinated species, is generally used in this process. Often, a third or fourth line will be included in the pedigree of a new line through crossing, as the initial two parent lines will frequently not contain all of the characters desired in a new line. During generation advance from F.sub.1 to F.sub.2, F.sub.2 to F.sub.3, F.sub.3 to F.sub.4, etc., homozygosity increases with each generation as the plants naturally self-pollinate, so that typically in six (6) generations, the individual plants are 98.5% homozygous.
Backcrossing is a breeding technique that can be used to improve an inbred line. Backcrossing transfers a specific desirable trait from one line or source to a line that lacks that desirable trait. This can be accomplished, for example, by first crossing a superior line S, which is a recurrent parent, to a donor line D, which is a non-recurrent parent that carries the appropriate gene(s) for the trait in question. The progeny of this cross is then mated back to the superior recurrent parent S followed by selection in the resultant progeny for the desired trait to be transferred from the non-recurrent parent D. After five or more such backcross generations with selection for the desired trait, the progeny will be homozygous for loci controlling the characteristic being transferred, but will be like the superior parent for most or almost all other genes. After this transfer of the desirable gene(s), some selfing (self-pollination) and plant selection may be required to reach homozygosity and population homogeneity. Alternatively, genetic markers can be used to select for the genotype of the recurrent parent S, and either molecular markers or plant phenotype can be used to select the desired genes from the non-recurrent parent D. With adequate markers, 2 to 3 generations of backcrossing should be adequate to return to the recurrent parent S genotype with the desired loci added from the non-recurrent parent D.
Once a single plant is homozygous at all or nearly all loci, a variety can be produced from this single plant through simple generational advance, with care being exercised to control external pollen flow and continued vigilance exercised in removing any apparent off-types in the population.
The development of Cytoplasmic Male Sterile ("CMS") hybrids in rice involves additional steps beyond the development of varieties. This hybrid system, also known as the "3-line system," is used in many crop species, including sorghum, wheat and onions. It requires, as the name implies, three (3) different lines for the complete system. These are generally called A-lines, B-lines, and R-lines. Such a three line system is described further below.
In developing hybrid rice lines, it is essential to utilize parent lines which, when combined, will give the required quality and yield characteristics that meet market standards. Secondly, both parents must have seed production performance which will result in an economically viable hybrid. Most importantly, rice hybrids must fit farmers' production systems without requiring extensive operational changes.
To accomplish the above goals, it is often essential to produce and test many hundreds of hybrids over many locations for several years to locate those rare parental combinations which meet these criteria. With several hundred important genes segregating, it is difficult to find good parent lines. Then, with, dominance, co-dominance, over-dominance, and epistasis modifying the simple interaction of the two parents, selection of parent lines for good hybrids is doubly difficult. From estimates of the number of plants involved throughout selection of R- and B-lines, and in the backcrossing of the A-line, it is likely that more than 1.times.10.sup.6 genetically distinct plants will have to be examined and rejected before a single hybrid is ready for the marketplace.